The use of contralateral prophylactic mastectomy (CPM), which is removal of the healthy breast in a woman with unilateral breast cancer, has risen five-fold in the past 15 years. Although a case can be made for CPM among women with a strong family history of breast cancer, CPM is not recommended for women diagnosed with sporadic breast cancer. The dramatic increase in CPM is due to women with sporadic breast cancer choosing CPM. There are a number of reasons women at sporadic risk choose CPM. It is thought that CPM is considered more acceptable due to technical advances that have resulted in improved breast reconstruction outcomes and because high profile celebrities chose CPM. The primary reason women choose CPM is that breast cancer triggers considerable distress and uncertainty about whether breast cancer may return, either to the healthy breast or to another part of the body. This may motivate patients to choose the more aggressive treatments regardless of their risks and benefits. Uncertainty about cancer recurrence may also motivate women to choose CPM to reduce this worry, even when women realize they are not at significant risk for a cancer in the other breast and that CPM will not impact their survival. Patients may be less likely to heed their surgeons' advice that CPM is not medically indicated. There is a clinical need for greater patient education regarding the decision to undergo an aggressive and permanent intervention that is accompanied by significant morbidity and potential complications. The decision to have CPM among women with sporadic breast cancer places patients' desires for relief of anxiety and uncertainty above data suggesting that these women do not have a significant risk for cancer in the healthy breast and the fact that CPM does not confer a survival benefit. In situations when patients face complex decisions where they need to weigh the risks and benefits of choices, decision support aids (DA) can be helpful to promote a well-informed choice. Toward this end, we propose to develop a theoretically guided, web-based CPM-DA for women with sporadic breast cancer who are considering CPM to promote informed decision making. We will accomplish this in two phases. Phase 1 is a developmental phase where the DA content is developed based upon interviews with women who have undergone or chosen not to have CPM. These data will be analyzed using qualitative methods. Next, the team will develop the CPM-DA, gather feedback about it from patients, and finalize the CPM-DA. Phase 2 will be a small randomized clinical trial of the CPM-DA comparing it with Usual Care among 80 women considering CPM. The primary aim is to evaluate the feasibility and acceptability of the CPM-DA. The secondary aim is to provide preliminary data on the impact of the CPM-DA on preparedness to make the decision about CPM, CPM knowledge, self-efficacy for managing worry and surveillance, perceived cancer risk, worry about recurrence, decisional conflict, and CPM intention.